Conventionally, a technology of an optical coherence tomography (OCT) attracts attention as state-of-the-art medical diagnostic technology which is non-invasive and harmless to a human body, and equipment development and application research to living body measurement have been promoted energetically. OCT is a technology of acquiring the information on an objective optical response structure to the depth where light can approach from the objective surface, and is applied to the examination of the fundus etc. The OCT, which is proposed from the beginning and put into practical-use, uses a laser light which is a light beam having a narrowed spot size. The laser light is divided into an illumination light and a reference light. The illumination light is entered into an object, and a light which reflects and returns out of the object is observed by interference with a reference light. Information on a reflective position of the light in the object or on reflective intensity, i.e., information on structure of the object in a direction of the light propagation (the longitudinal direction, i.e., the depth direction), is obtained from the observed interference. If the position on the object surface in which laser light is entered is moved in one dimension, a tomographic image of one plane will be obtained. If the position is made to move in two dimensions, three-dimensional structural data of the object will be obtained, and the tomographic image in an arbitrary tomographic plane can be reconstructed from the data. In this case, the tomographic image is a map showing planar distribution of average intensity of the reflected light in a thickness direction of a layer with finite thickness. When the layer with finite thickness is represented by one plane (for example, a central plane of the layer), the representative plane is named a tomographic plane or a reconstruction plane.
OCT is categorized roughly into two by the method deriving the information in a longitudinal direction. One is time domain OCT (TD-OCT) which obtains the flight time of a light pulse directly, and the other one is Fourier domain OCT (FD-OCT) which obtains difference of distance in a longitudinal direction from spatial frequency of interference fringes. The former TD-OCT processes the interference of a light wave in a real space (time domain). TD-OCT is OCT put in practical use first, and only the information on one point in a depth direction is acquired by one irradiation of illumination light. Therefore, to acquire the information on each point in the depth direction by TD-OCT, it is necessary to change the light path length of a reference light. Therefore, a reference mirror on a optical path is moved mechanically. The latter FD-OCT processes the interference of the light wave in a Fourier space (a frequency domain or a wavelength domain). FD-OCT is further categorized roughly into spectral region OCT (SD-OCT) which uses a wavelength fixed light source and a spectroscope, and wavelength-scanning type OCT (SS-OCT) which changes the transmitting wavelength in a light source. This FD-OCT does not need mechanical displacement of the reference mirror, and realizes improvement in the speed of an image pickup.
However, since a laser light of a narrowed beam spot size is used in any type of the FD-OCT, it is necessary to carry out a Galvano scan or to scan mechanically a movable head consisting of a reference mirror and an optical interferometer in one dimension or in two dimensions along the object surface for obtaining two-dimensional or three-dimensional data, and improvement in the image pickup speed is limited. On the other hand, as an image pickup method which does not need the mechanical scan of an optical system, a tomographic image pickup method by a digital holography using an imaging lens and an wavelength-scanning laser light is proposed (for example, refer to Non-Patent document 1). Moreover, an example in which this tomographic image pickup method is applied to a living organization has been reported (for example, refer to Non-Patent document 2). The tomographic image pickup method, described in these Non-Patent documents 1 and 2, uses a wavelength-scanning plane wave light as a illumination light, and records an object light on a hologram for every wavelength. A hologram for reconstructing a tomographic image on a reconstruction position is derived by obtaining a phase of an object light for every wavelength in a common reconstruction position and by adding up each of holograms which are normalized using each of the object light phases obtained. Tomographic images in other positions are derived by propagating the light wave recorded on the hologram.
The digital holography has been developed as a recording method for carrying out a high-speed image pickup. For example, an one-shot digital holography is proposed (for example, refer to Patent document 1), which can record a wide-band complex amplitude hologram correctly at high speed by applying a space heterodyne modulation and a spatial frequency filtering to off-axis holography. Moreover, to solve the problem of the conventional optical microscope, using this one-shot digital holography, a holographic microscope, a method for recording a hologram image of a microscopic subject, a method for generating a hologram for high-resolution image reconstruction, and a method for reconstructing an image are proposed (for example, refer to Patent document 2). This microscope is a penetrating and also reflecting type microscope, and a lensless holographic microscope without using an imaging lens This microscope can solve the problem of the conventional optical microscope which is affected by an influence of a medium or the imaging lens. That is, by not using the imaging lens, this microscope can record an object light of large numerical aperture at a one-shot correctly, and can reconstruct exact and non-distortion high resolution three-dimensional movie using a computer.